The present invention is the nucleotide sequence encoding a 52 kDa Ro/SSA autoantigen found in some systemic lupus erythematosus and Sjogren's syndrome patients, the encoded protein and methods for use thereof in diagnostic and therapeutic applications.
Systemic lupus erythematosus (SLE) is similar to many other disorders in which autoantibodies are found and thought to be important in etiology and pathogenesis. SLE can be grouped with those diseases that commonly have autoantibodies present but for whom a central role of autoantibody in pathogenesis leading to clinical expression has yet to be fully established or accepted. Other such diseases include Sjogren's syndrome, rheumatoid arthritis, insulin-dependent diabetes mellitus, primary biliary cirrhosis, Wegener's granulomatosis, inflammatory bowel disease, polymyositis, dermatomyositis, scleroderma, and many others.
Typically, autoimmune diseases present with a wide array of symptoms and clinical signs. The production of circulating autoantibodies to ribonucleoprotein complexes (RNPs) is a unifying characteristic of some of the rheumatic autoimmune diseases. The most common antigens in SLE and closely related disorders include: Ro/SSA, La/SSB, nRNP and Sm. Initially, these antibodies were found using double immunodiffusion, but more recently sensitive solid phase assays have been developed to quantitate the autoantibodies. The Ro/SSA RNA-protein particle has been found to be a constituent of all human cells evaluated to date. Approximately half of Sjogren's syndrome and systemic lupus erythematosus (lupus) patients have anti-Ro/SSA precipitins. Approximately 75% of patients with subacute cutaneous lupus erythematosus or complement component C2 deficiency with SLE have anti-Ro/SSA precipitins, and virtually all patients with C2 or C4 deficiency have elevated levels of anti-Ro/SSA when measured by ELISA. Over 80% of mothers of newborns with neonatal lupus dermatitis or complete congenital heart block have these autoantibodies. As many as 5% of patients with rheumatoid arthritis, polymyositis, and progressive systemic sclerosis have anti-Ro/SSA, as reported by R. M. Bernstein, et al., Mol. Biol. Med. 2:105-120 (1984); and J. B. Harley and K. K. Gaither, Autoantibodies. In Rheumatic Disease Clinics of North American: Systemic Lupus Erythematosus 14:1, 43-56 (1988).
It has also been shown that some normal individuals have low levels of anti-Ro/SSA, that some normal family members of SLE patients have anti-Ro/SSA, and that 1% of normal pregnant women, and 0.1% of a cohort of hospitalized patients have precipitating levels of this autoantibody (K. K. Gaither, et al., J. Clin. Invest. 79:841-846 (1987); T. J. A. Lehman, et al., J. Rheumatol. 11:644-647 (1987); M. Calmes and B. A. Bartholomew, J. Clin. Pathol. 38:73-75 (1985); P. J. Maddison, et al., J. Rheumatol. 5:407-411 (1978)). Even if the anti-Ro/SSA autoantibody is not pathogenic, the concentrations of anti-Ro/SSA autoantibody achieved by patients can be extraordinary, and is commonly higher than 1 mg/ml of specific anti-Ro/SSA immunoglobulin (K. K. Gaither and J. B. Harley, Prot. Biol. Fluids Proc. Collog. 33:413-416 (1985); J. B. Harley, et al., Arthritis Rhuem. 29:196-206 (1986)). The immune system derangement leading to this specific overproduction of anti-Ro/SSA is not apparent but is likely to reflect a fundamental mechanism related to the immunopathogenesis of the related diseases.
Ro/SSA has been referred to by several other names, including "SSA/Ro", "SS-A/Ro", "SS-A", "Ro", and "Ro(SSA)". Historically, the biochemical characterization of the Ro/SSA complex has centered around a 60 kDa protein associated with one of four hY RNAs, ranging from 80 to 112 bases, although the antigenic reactivity of the complex appears to be independent of the RNA. The Ro/SSA family of proteins has now been shown to have several molecular forms which are operationally defined by the molecular weight of the antigen identified. As reviewed by Ben-Chetrit, et al., in J. Exp. Med. 167, 1560-1571 (1988), the protein components of Ro/SSA have been described as polypeptides having molecular masses ranging from 50 to 150 kiloDaltons (kD). A major form has an apparent molecular weight of 60 kDa. Recently, two additional proteins bound by anti-Ro/SSA sera have been identified by M. D. Rader, et al., J. Clin. Invest. 83:1556-1562 (1989), with molecular weights of 52 kDa and 54 kDa. Ben-Chetrit, et al., (1988) also report a 52 kDa protein. Chan, et al., reported at the Molecular and Cell Biology of Autoantibodies and Autoimmunity, First International Workshop, Jul. 27-29 (1989), that they had cloned a gene encoding a 46 kDa protein reactive with antisera against a 52 kDa Ro/SSA protein that was distinct, based on sequence comparison, from the cDNA of 60 kDa Ro/SSA. Other groups have confidentially reported that they have isolated cDNA encoding a different 60 kDa protein, having a molecular weight predicted by sequence analysis of 48 kDa.
It is impossible to determine at this time how many different autoantigens are produced which form complexes with RNA and that are characteristic, or involved in the pathogenesis, of autoimmune disorders in humans, such as SLE and Sjogren's syndrome. The proteins may vary not only from patient to patient, but in cellular origin. For example, in nucleated cells, 60, 52 and perhaps 45 kDa forms of the Ro/SSA protein have been found using Western blot analysis. Certain lupus patient sera contain antibodies which recognize only the 60 kDa form, others only the 52 kDa form, and others have antibodies which bind to both the 60 and 52 kDa forms. In red blood cells, 60 and 54 kDa proteins have been identified in Ro/SSA particles. It appears that these proteins can only be identified with any certainty by comparison of nucleotide and amino acid sequence comparison.
It is therefore an object of the present invention to provide cDNA encoding an autoantigen characterized by a molecular weight of approximately 52 kDa to which serum antibodies are produced by certain patients having autoimmune disorders.
It is a further object of the present invention to provide the cDNA and the protein encoded by the cDNA and methods for use thereof for diagnostic and therapeutic purposes.